The Psychology of Quality and More
One of the most important concepts in lean manufacturing is that of ‘push’ and ‘pull’ systems. Traditional manufacturing uses ‘push’ systems, where the goal is to ensure all people and equipment are optimally used. A result of this is that where there is a chain of operations (stamping, milling, assembly, etc.) then if a earlier operation is faster than a later operation, then a pile of inventory will build up in front of the downstream machine, as in the diagram below.
Inventory is, effectively, waste. It takes up space, uses up working capital and is liable to damage and devaluation. The ultimate inventory pile is at the end of the assembly process, where finished goods are stacked up in warehouses, waiting for customers to buy them.
The principle of ‘pull’ is that control is transferred from the beginning of the line to the end. Thus, in the example above, Operation 2 needs to control what Operation 1 gives them. The secret of this is the ‘Kanban card’. A kanban card is a control device which effectively says to the recipient ‘Give me N items, and N items only. When you have done that, stop! Wait until you get the next kanban card.’ The diagram below now shows the changed conversation between Operation 1 and Operation 2.
Operation 1 now may stand idle for a while, which may seem like a cost, but it is not as great as the cost of idle inventory.
The trick with pull systems is to see how small a number of items you can pull with a kanban. You will need a certain number to manage natural variation in the process, but you will not need any more. Reducing the number of items pulled, even by one, is an effective way of highlighting problems and bottlenecks in the system. For example, reducing the kanban number in the example to five may lead to Operation 2 being idle sometimes, but if some process improvement is done on it, it may even be able to handle as low as four kanban items.
And worth remembering is that transferring inventory out of a manufacturing plant can be accomplished with a logistics company. Lesaint.com specializes in this kind of third-party logistics (3PL).
Next time: Value Stream Mapping
This article first appeared in Quality World, the journal of the Institute for Quality Assurance
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